The main absorbent part of most disposable sanitary products is the pad, or core. The pad is often made of wood pulp that has been fiberized by a special mill, designed to handle fluffing pulp. After the pulp is fiberized, it is drawn out of the mill onto a moving air forming screen, which is normally between 40-80 mesh. The pad is formed on the screen in a forming chamber in which the fluffing pulp is placed on the forming screen and is forced into a compact configuration by suction of air through the screen. After the pad is formed on the screen, it moves through a set of profiling rolls and on to the folding and packaging part of the converting machine.
The air that is pulled through the forming screen from the pulp mill contains small amounts of fiberized fluff pulp. Experience has shown that the amount of pulp that comes through the forming screen is 2% or 3% of the total amount of fluff that enters the forming chamber.
Several filters have been developed for filtering the pulp out of the air exiting tile forming chamber. These filters have several advantages. First, the filters clean the air that comes from the forming screen and return the air to the plant area or deposit the air outside of the plant.
Second, the fluff that comes through the forming screen is returned to the mill or forming chamber. The fluff that is returned to the process represents a substantial cost savings to the manufacturer.
For uniform pad formation to take place in the forming chamber, the volume of air moving through the forming chamber and the pressure of that air should be consistent. If the air volume or pressure is changed, the pad will have different thicknesses and absorbencies and will not reach specification. By assisting in moving air through the forming screen a properly-constructed filter can help to assure air volume and pressure consistency through the forming chamber.
One of the filters that is being used for removing the pulp fluff from the air float moves through the forming screen is the rotary drum variety, such as is depicted in FIG. 1. As can be seen in FIG. 1, the process air from the production machine, that is, the air that is moving through the forming screen from the pulp mill, is fed through a conduit 12 into the drum filter enclosure 14. A rotary drum 16, which includes a filter media 18 along its outside, rotates within the drum filter enclosure 14. One end of the rotary drum 16 is closed off, and the other end of the drum opens to a compartment for withdrawing the air. One or more fans are used to move air through the open end of the drum 16 through the media.
As the drum 16 rotates and the clean air is pulled through the media 18, pulp fluff 19 settles on the media. This pulp fluff is vacuumed off through a suction nozzle 20 by a purge fan 22. This fan 22 and other conduit 24 then routes the fluff 19 back to the production line. The clean air which is pulled through the media 18 by the fans of the system is returned to the plant area itself or is exhausted outside the plant.
The utilization of a prior art rotary drum filter, such as is shown in FIG. 1, with a forming chamber helps to create uniform air pressure and volume at the forming chamber. The forming chamber and drum filter utilize at least one fan to create suction at the forming screen. A clean air fan is used at the end of the rotary drum filter for moving the fluff through the media on the drum filter and for creating suction for moving the fluff from the production machine and through the forming screen. In addition, a material handling fan may be used to move the forming air and fluff from the mill through the forming chamber. The material handling fan, also known as a forming fan, would be located on the line extending from the forming chamber to the drum filter.
The drum filter enclosure generally can only handle approximately 12 inches water column (wc) of negative pressure. The material handling fan must be used if the forming chamber requires more than 8 inches wc of negative pressure. If a material handling fan is used in the forming chamber, then the fan at the rear end of the drum filter, or the clean air fan, is used as a balancing fan to keep the filter under a negative pressure. Because increasing forming chamber pressure is a common requirement of sanitary products machine manufacturers, material handling fans are often used to generate the required high pressures and volumes in a system. In such systems, the clean air fan located at the end of the rotary drum filter serves mainly as a balancing fan to keep the filter under negative pressure.
Prior art processes utilize one drum filter for one production line. The drum filters are expensive and take up a large amount of floor space in a plant. There is a need for a rotary drum filter system which is capable of use for more than one line.
Simply running two production lines into a drum filter to handle twice as much fluff has not been found to be an adequate solution to the problem. By running two production lines to one rotary drum filter, uniform air volume and pressure will not be maintained if either of the connected production lines cease to operate. This problem is best explained by reference to FIG. 2 and the description in the following paragraphs.
The system set forth in FIG. 2 discloses a first line A having a first forming chamber, a second line B having a second forming chamber, and a drum filter C. Forming fans are located on both forming chambers to provide necessary air volume and static pressure for pad formation and so as to deliver the forming air and small amounts of fluff to drum filter C. The filter C includes a balancing fan which serves to keep the system under negative pressure. The fluff that is returned from the drum filter is sent back to the forming chamber A.
If both forming chambers in the drum filter are in operation, the system operation works fine. Also, if line A stops running for a few minutes, the filter C can go into a recirculation mode, and fluff from A and B will be recirculated into the filter C. Under this condition, the entire system should run fine for a short period.
If line A stops for a long period of time, such as one hour or more, and the forming fan for that line is stopped to keep from wasting electricity, the air volume to the filter C is dramatically affected. This causes a series of problems. First, the forming fan on line B is putting fluff into the filter C, but the fluff is not going out of the filter C because fluff is set up to return to line A, which is no longer running and, therefore, cannot use the fluff. The reclaim fluff cannot be put into line B because it is not set up to take the reclaim. Rerouting the fluff to line B takes time and an unnecessary shutdown of line B, in addition to the already shutdown line A. Furthermore, moving the reclaim fluff to line B would also take an adjustment in the forming chamber of line B for the additional fluff which is entering the chamber.
If, on the other hand, line B stops for a few minutes, fluff no longer goes from line B to the drum filter, cutting the amount of fluff that is going to line A from the filter in half. This reduction affects the pad weight; therefore, the product is no longer within specification.
Another problem exists with these shutdowns. With either the forming fan on line A or the forming fan on line B out of operation, the air volume is reduced to the balancing fan of the system. In most cases, this reduction means that the balancing fan is not getting enough air for its speed, causing the fan to become unstable. Instability will cause the fan to vibrate and shakes the filter enclosure and connecting duct. Such vibration may result in mechanical failure of the balancing fan and/or damage to the drum filter.
There is a need in the art for a rotary drum system which can be used for more than one fluff forming line and which can continue that use after one of the lines is shut down for a short or long period of time. Preferably, such a system would handle reclaimed fluff widen a product line stops operation and would be able to handle the change in air volume and static pressure to the balancing fan when forming air from one production line no longer goes to the drum filter.